The conventional bolt-hole cap for a bolt hole in a rail is disclosed in FIGS. 13 and 14, and generally comprises a slider D movably mounted to a rail E, and the slider D is endlessly moved along the rail E. Multiple bolt holes F are located at the rail E and each has a bolt G extending therethrough so as to fix the rail E to a fixed body H. When the slider D moves along the rail E, the slider D has an end seal connected thereto which is matched on the rail E to prevent foreign objects from entering the slider D to damage the slider D. However, the dust or foreign objects are easily hidden in the bolt holes F of the rail E. When the slider D passes over the bolt holes F, the dust or foreign objects can enter the slider D such that the life of the slider D will be reduced. Therefore, a bolt-hole cap C is used to seal the bolt hole F to prevent the dust or foreign objects from accumulated therein.
The diameter of the bolt-hole cap C is slightly larger than the diameter of the bolt hole F, so that when the bolt-hole cap C is engaged with the bolt hole F, the periphery of the bolt-hole cap C is snugly engaged with the inside of the bolt hole F. Furthermore, when the bolt-hole cap C is engaged with the bolt hole F, a gap/distance is formed between the bolt-hole cap C and the bolt C, and hence the bolt-hole cap C is positioned only by friction. When a force is applied to the bolt-hole cap C and overcomes the friction, the bolt-hole cap C shifts. When the end seal passes over the bolt hole C, the end seal scrubs the rail E and generates a foreign object lump which is pushed downward to the bolt-hole cap C so that the bolt-hole cap C is slightly lowered into the bolt hole F, and an extra space N is formed and more foreign objects are accumulated in the space N.
JP2007-321769 discloses a bolt-hole cap which has multiple rings on the outer periphery thereof and the diameter of the rings are larger than that of the bolt hole, such that friction between the bolt-hole cap and the bolt hole is enhanced. When the bolt-hole cap is hammered into the bolt hole, the bolt-hole cap is securely engaged with the bolt hole. However, during the hammering action, the top surface of the bolt-hole cap may not be in flush with the surface of the rail due to uneven hammering force.
JP2009-127729 discloses a bolt-hole cap which has gradually narrowed protrusion on the inside thereof, the protrusion extends from its base portion to its distal end gradually thinner and eventually forms a cone-shaped end. The protrusion is a ring-shaped part and is located at a distance from a ring-shaped portion. When the bolt-hole cap is inserted into the bolt hole, the distal end of the protrusion contacts the bolt and is deformed radially so that the protrusion contacts the bolt. By the contact between the protrusion and the bolt, the space between the bolt-hole cap and the bolt is filled. However, the distal end of the cone-shaped part cannot provide sufficient force even the cone-shaped part is deformed. When the bolt-hole cap is applied by a large force, the distal end will further be deformed radially to drag the bolt-hole cap into the bolt hole so as to form an extra space.
DE1020004001944 discloses a bolt-hole cap as shown in FIG. 15, wherein a recess L is formed at the top of the bolt hole F and the bolt-hole cap C is engaged with the recess L. The bolt-hole cap C is supported by the inside of the recess L to keep the top surface of the rail E to be in flush with the top face of the bolt-hole cap C. The bolt-hole cap C can be made by metal to be more durable from friction. In order to keep the top surface of the rail E to be in flush with the top face of the bolt-hole cap C, the tolerance between the recess L and the bolt-hole cap C has to be less than the normal tolerance, such as 0.02 mm. However, the less the tolerance is maintained, the higher manufacturing cost is occurred.
The conventional way to position the bolt-hole cap in the bolt hole is by force-fitting the bolt-hole cap and the friction. It is noted that the friction is not sufficient for position the bolt-hole cap which tends to be lowered into the bolt hole by a downward force. Even the bolt-hole cap with supporting means has similar problem because the supporting means cannot provide sufficient force and the support means is further deformed. Therefore, the bolt-hole cap is moved into the bolt hole. The recess defined in the top of the bolt hole provides good support to the bolt-hole cap, but the cost is high due to higher standard for machining. In order to secure the bolt-hole cap in the bolt hole, prevent foreign objects from entering the rail, and reduce the manufacturing cost, a high strength bolt-hole cap with low manufacturing cost is needed.